Wireless networks have become increasingly popular, as computers and other devices can be coupled for data communications without requiring wired connections between the network nodes. One set of standards for wireless networks is the IEEE 802.11 standards, but other wireless standards or protocols might be used instead. In the IEEE 802.11 standards, there are at least two widely-used standards, 802.11a and 802.11b, and communication systems and devices might be required to support both standards and/or be required to operate in areas where both are being used.
Interference among the two standards can be avoided, as they operate in different frequency ranges. However, recent additions such as the 802.11g standard allow for OFDM transmissions (802.11a is an OFDM transmission protocol) in the 2.4 GHz band where 802.11b direct-sequence spread-spectrum transmissions might be present. The fact that packets with different modulations can be present within the same network creates difficulties in the design of 802.11 packet detection circuits, as they have to detect the presence of a packet with a fraction of its preamble and it has to indicate whether the packet is an 802.11a packet or an 802.11b packet with a very low probability of error.
Prior to decoding bits having information content provided by a transmitter, a receiver typically senses a packet being transmitted and then performs steps to characterize the channel, synchronize with the transmitted packet, and the like. Packet detection is a process of determining that a packet is present on the channel (i.e., it is being, or has been, transmitted by a transmitter), determining the type of packet it is (at least to the extent needed to perform further processing on the packet or its contents) and to activate receiver components as needed to handle further processing. In some receivers, the receive logic is implemented in digital signal processing (DSP) commands provided to a DSP processor. Where the logic that implements data processing of received packet data is implemented as instructions, those instructions can remain unexecuted until a packet detector indicates that the received signal contains a packet to be further processed. Where the logic that implements data processing of received packet data is implemented as hardwired circuitry, the receiver can be configured to remove or lower the power to such hardwired circuitry until a packet detector indicates that the received signal contains a packet to be further processed. In either case, processing power and/or computing effort is preserved when no packets are detected. This saves power and/or processing requirements, which are often constrained in wireless receivers, but requires packet detection. So that portions of the packet are not lost, the receiver should detect and existence of a packet and take any actions needed to start packet processing before essential elements of the packet are missed. Therefore, packet detection should be efficient and quick.
In addition to the problem of possibly overlapping 802.11 signals, an 802.11 receiver must also deal with narrowband non-802.11 signals, such as Bluetooth, scientific equipment, medical equipment or microwave ovens, and the receiver's packet detector should preferably not cause false triggers on such interference. Aside from the interference issues and packet detection issues, a receiver that must receive and process both 802.11a and 802.11b signals cannot use a simple common sampling scheme, as conventional 802.11a receivers operate at a 20 MHz sampling rate, while conventional 802.11b receivers operate at a sampling rate of 22 MHz.
A typical node in a wireless network includes a receive chain and a transmit chain and each chain uses only one antenna at a time. However, with multiple input, multiple output (MIMO) communication systems, more than one transmitter antenna and/or more than one receiver antenna is used, with each transmitter antenna possibly transmitting different bitstreams as other transmitter antennas and each receiver antenna preferably receiving at least a slightly different input from the channel than other receiver antennas.
MIMO communication systems are known in the art. Such systems generally include a transmitter having a number (Mt) of transmit antennas communicating with a receiver having a number (Mr) of receive antennas, where Mr and Mt may or may not be equal. In some keying schemes, bits of data to be transmitted are grouped and each group of bits is mapped to a symbol (a particular combination of phase and amplitude) in a signaling constellation. A number of constellations are known in the art, including binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM) constellations. In a MIMO communication system, each of the Mt transmit antenna transmits, at substantially the same time, a symbol representing a different group of bits. Thus, if each symbol represents B bits, the number of bits transmitted per channel “period” is B*Mt.
Each receive antenna receives a signal that is a combination of signals from the transmit antennas, modified by channel properties (e.g., fading and delay) and noise. The receiver decodes (i.e., reconstructs) the Mt transmitted signals from the Mr received signals using its knowledge of the possible transmitted symbols and the properties of the communication channel. Because of the improved reception abilities of multiple antenna systems, they are often expected to receive signals with lower signal-to-noise ratios (SNR) than other systems. With a wider expected operating range in terms of SNR, correct packet detection is expected at lower SNRs, making many conventional packet detection schemes unsuitable.
It would be desirable to overcome the shortcomings of the prior art described above.